In the healing arts there is often a need for an implant material to replace, repair, or reconstruct hard tissue in the body of a patient. For example, hard-tissue implant materials are used in medicine and veterinary medicine as a prosthetic bone material to repair injured or diseased bone. Hard tissue implant materials are also used in the construction of prosthetic joints and to fix prosthetic joints to bone. In dentistry, hard tissue implant materials are used in the construction of prosthetic teeth and tooth roots and to replace or augment the edentulous ridge.
U.S. Pat. Nos. 4,535,485 (the '485 Pat. No.) and 4,536,158 (the '158 Pat. No. ) disclose certain implantable porous prostheses for use as bone or other hard tissue replacement which are comprised of polymeric materials. The prostheses of the '485 and '158 Pat. Nos. are composed generally of polymeric particles. The particles have an inner core comprised of a first biologically-compatible polymeric material such as polymethylmethacrylate and an outer coating comprised of a second biologically-compatible polymeric material which is hydrophilic, such as polymeric hydroxyethylmethacrylate. The particles may incorporate a radio-opaque material to render the particle visible in an X-ray radiograph. The particles may be bonded together to form a unitary structure which can be implanted in the body. Alternatively, a mass of the particles may be implanted in the body in an unbonded, granular form. In either the bonded or the unbonded form, interstices between the implanted particles form pores into which tissue can grow. The hydrophilic coating on the particles facilitates infusion of body fluids into the pores of the implant, which facilitates the ingrowth of tissue into the pores of the implant.
Although porous prostheses of the '485 and '158 Pat. Nos. have proven to be satisfactory for many applications, there is room for improvement. For example, when it was attempted to graft together two bone fragments separated by a gap within the body of a rat by packing a mass of the polymeric particles in granular form in the gap, it was found that the tissue which grew into mass of particles was essentially cartilage and a dense fibrous connective tissue. Substantially no bone tissue formed in the gap packed with the particles between the two bone fragments. The joint between the two bone fragments was unacceptably fragile and nonrigid.
An article by Denissen and deGroot published in the Journal of Prosthetic Dentistry, volume 42, pages 551-556, (November, 1979) disclosed that implant materials based on calcium phosphate salts should be biocompatible and capable of forming tight bonds with surrounding bone. The article noted that previously available calcium-phosphate-based materials could be obtained only in a biodegradable form, which, although useful for bone replacement, was not useful for tooth-root implants. Denissen and deGroot disclosed that a biostable tooth-root implant could be prepared by first preparing a porous implant form of biodegradable, sintered calcium hydroxylapatite and then coating and impregnating the implant form with polymeric poly (hydroxy-ethyl- methacrylate).